The concert will take place in front of 15,000 pop fans at the MEN Arena in Manchester on 14 May.

Vote by text

It's raising money for a charity called Nordoff-Robbins Music Therapy, which helps people with disabilities and illnesses.

The charity has a clinic in London but this money will help pay for a planned centre in Manchester.

Fans will be able to vote by text during the actual gig to decide which songs the bands play.

'It touched us'

Jenny from Atomic Kitten told Newsround's website: "We chose this charity because of the music connection, we really want to get involved. To set up a centre in Manchester will be a great thing for us because it's near our home towns."

Duncan from Blue said: "We went to the (Nordoff-Robbins music therapy) centre one and a half years ago. What we saw touched us, the way music can help. It doesn't matter how disabled these people are, the fact that music can help really touched me"

And the bands had a few ideas of their own about the one-off gig.

Lee from Blue said he wouldn't want to sing Cheeky Girls, but Jenny said that she'd pay a lot to see Blue in hot pants!